Happy Sci-Day, fans! Today, I will be finishing up the Evolutionary History of Dinosaurs blog mini-series with a post about the major lineage of dinosaurs known as the Ornithischians. Now, I know some of you might be wondering why I'm not doing a post about Sauropoda, but I do have a good reason. Since there were no sauropods in the Hell Creek formation, spending a lot of time going into their history and anatomy might give people the wrong idea of what animals will be represented in the game itself.
The Ornithischian-Saurischian split is the earliest divergence in the entire Dinosauria clade. In the Ornithischian lineage, the bones of the pelvic girdle were configured such that both pubis and ischium faced caudally, whereas the Saurischian clade retained the ancestral pelvic bone configuration (though several Saurischian lineages would evolve the same configuration through convergence). Another feature that separates Ornithischians from Saurischians is the presence of an additional bone extending from the dentary, called the predentary. This bone did not bear teeth, and coincides with the premaxilla of the upper jaw to form a beak-like structure that would be useful for eating plants.
The earliest confirmed Ornithischian is a species known as Pisanosaurus mertii, from the Carnian- Norian stages of the Triassic period (between 228 and 216 million years ago) of what is now Argentina (Butler et al., 2008). It was a small, bipedal dinosaur, approximately 1 meter in length and weighing around 2.27-9.1 kilograms (Holtz, 2008), though the incompleteness of the specimen makes such estimates somewhat uncertain. Like all Ornithischians currently known, it was herbivorous.
By the early Jurassic, the major clades of the group had already appeared. One such group is the armored Thyreophorans. This lineage would lead to two very famous lineages - the Stegosaurs and the Ankylosauria. Ankylosauria had extremely thick armor, a low-lying posture, and evidence indicates that they also had thick, muscular tongues (Hill et al., 2015). This group would split into the Nodosaurs and Ankylosaurs. In the Ankylosaurs, the scutes at the tip of the tail fused into a massive, bony club, whereas the prominent feature in Nodosaurs were their spikes. Additionally, the Ankylosaurs had large squamosal plates projecting from the bottom and top of the skull on both sides, a feature absent in Nodosaurs. Both Nodosaurs and Ankylosaurs were represented in the Hell Creek ecosystem, in the forms of Denversaurus (=Edmontonia?) schlessmani and Ankylosaurus magniventris, respectively.
Another of the major groups of Ornithischians was the Ornithopods. This group would lead to the extremely successful Hadrosaurs, or "duck-billed" dinosaurs, as well as several other lineages. One lineage of Ornithopod present in the Hell Creek formation was Thescelosaurus. The popular hadrosaur Edmontosaurus annectens also lived in the area.
Sister to the ornithopods were the marginocephalians. This lineage would split to form the Pachycephalosauria and the Ceratopsia. The former of the two contained famous creatures such as Pachycephalosaurus wyomingensis, whereas the latter contained species like Triceratops horridus. The Pachycephalosaurs were bipedal creatures with thick, bony, skulls varying in shape from round to relatively flat 'domes'. It has recently been proposed that the skull morphology thought to be indicative of different species may actually represent growth stages in a single species (Horner and Goodwin, 2009).
Ceratopsians also had very interesting carnial ornamentation, though it came in the form of large frills and horns of varying shapes and sizes. While the earliest members were bipedal with little to no frill or horns, later species would develop extensive frills and large horns, as well as a quadrupedal gait. Scale impressions are known from several species, including Triceratops, which show large, non-overlapping scales. Like in Pachycephalosaurs, there is evidence of complex cranial ontogeny, with specimens of Triceratops of different ages show significant differences in frill and horn shape. In fact, it has been suggested that Torosaurus latus is not a unique species, rather representing adult Triceratops (Scannella and Horner, 2010). While this assertion is by no means settled, the plasticity of the skull as evidenced by known specimens does not eliminate the possibility.
Well, I hope this Sci-Day has been informative! Be sure to tune in next week to learn more about the amazing creatures that lived on ancient earth!
Acknowledgements:
Butler, Richard J; Upchurch, Paul; Norman, David, B. 2008. The phylogeny of the ornithischian dinosaurs. Journal of Systematic Palaeontology 6 (1): 1-40.
Holtz, Thomas R. Jr. 2008. Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages.
Hill, R. V.; D'Emic, M. D.; Bever, G. D.; Norell, M. A. 2015. A complex hyobranchial apparatus in a Cretaceous dinosaur and the antiquity of avian paraglossalia. Zoological Journal of the Linnean Society 175 (4): n/a.
Horner, J. R.; Goodwin, M. B. 2009. Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus. PLoS ONE, 4 (10): e7626.
Scanella, J; Horner, J. R. 2010. Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny. Journal of Vertebrate Paleontology, 30 (4): 1157-1168.
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